You remove your t-shirt and are immediately struck with an idea of what to start with, so proceed to take off your bra. Your breasts aren't inordinately large, but they're large enough that wearing a bra is an absolute necessity, and you have never been fond of wearing a bra. You never saw the point in flaunting your breasts, and really they were more of a burden you had to bear than an asset. Plus, if you shrank your breasts, you could easily cover it up by padding your bra, and no one would know the difference. Nodding to your reflection in the mirror, you take a breath and focus.
Staring at your chest, you try to imagine it flat as a washboard, but it's hard for you to get a specific image. Instead, your mind wanders to bodybuilder pecs, both male and female. It would be nice to have some muscle, but for it to be firm enough you could get away without a bra. You really can see it now...
Shaking your head, you bring your eyes back into focus...and realize your reflection has changed. Where once round elongated breasts hung from your chest, now you had quite sturdy squarish pectorals. In awe, you reach up with a finger to poke your nipple, and shudder as a tingle goes down your spine. They're definitely still sensitive, but now your torso could pass for that of a weight-lifting guy, especially with your shirt off. The goatee makes you look even more masculine, despite the femininity of your face underneath.
You pull your t-shirt on, then chuckle in delight. It really wasn't obvious, and there definitely wouldn't be any difference from your usual self if you wore a padded bra. However, then you removed your t-shirt again. The whole point of this exercise was to understand your powers. You have haphazardly managed to change yourself, but to have real control, you would have to be able to return yourself to your original form at will. Should you practice that now, or have some more fun with random changes before you get serious?